qemu-system-x86_64 G.qcow2 -m 4096 -smp 4 -no-acpi -enable-kvm
  -name system76 -device virtio-net,netdev=net0
  -netdev tap,id=net0,ifname=tap0,script=no,downscript=no,br=br0
  -vga qxl -spice port=5900,addr=127.0.0.1,disable-ticketing     &

remote-viewer spice://127.0.0.1:5900 # from the user (not root)

virt-viewer spice://127.0.0.1:5900 # from the user (not root)

Failed to connect: No virtual machine found

virt-viewer -c libvirtd:///system system76

Unable to connect to libvirt with URI libvirtd:///system